ΔE 13.3, fair: the same colour family, but not a straight swap. Rusty Metal is a little darker, more muted, warmer than Steampunk Copper.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Rusty Metal is a little darker, more muted, warmer than Steampunk Copper.
The same difference applies the other way round: Steampunk Copper is a little lighter, more saturated, cooler.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.
See Steampunk Copper on the colour wheel See Rusty Metal on the colour wheel
All matches: Steampunk Copper equivalents · Rusty Metal equivalents